The present invention is directed to heat transfer initiators for propellant, pyrotechnic, and explosive devices. In particular, the present invention is directed to initiators that utilize heat transfer to ignite an non-detonating autoignition material to act as a thermal switch to reliably and precisely control the time to function of propellant, pyrotechnic, and explosive devices, and may also be used to reliably and precisely control the time to function of such devices. The initiators of the present invention are particularly useful as through-bulkhead initiators.
Various initiators that are actuated by a pyrotechnic, electronic, or mechanical input are known in the art for the control of the function of propellant, pyrotechnic, and explosive devices. Initiators are used in a variety of applications, including, but not limited to, passive vehicular safety systems, fire suppression systems, rockets, and munitions. When actuated, the initiator provides a thermal output, typically, in the form of heat, hot gas, hot particulates, and/or flame. Actuation of a prior art initiator is typically achieved electrically, or mechanically.
In many applications, where a reliable electrical actuation signal is available, such as in vehicular air bag systems, a pyrotechnic squib may be used as an initiator. Pyrotechnic squibs such as those disclosed in U.S. Pat. No. 6,168,202 to Stevens, are well known in the art. A typical pyrotechnic squib includes a pair of electrical leads, connected by a bridge wire, which is in thermal contact with an ignition composition. Passing an electrical signal through the electrical leads and the bridge wire, heats the bridge wire, and ignites the ignition composition. The thermal output from the reaction or combustion of the ignition compound ignites a pyrotechnic material within the squib that provides the desired thermal output used to initiate function of a main propellant, pyrotechnic, or explosive charge. Pyrotechnic squibs will only function properly in applications where an electrical actuation signal is reliably available.
A mechanically actuated initiator is disclosed in U.S. Pat. No. 5,913,807 to Bak. The disclosed initiator uses a percussion primer, of the type used in bullets, which, when struck ignites a second charge that provides the desired thermal output. However, such mechanical actuation systems can be complicated and unreliable.
U.S. Pat. No. 3,945,322 to Carlson et al. discloses a through-bulkhead initiator for causing an explosion on one side of a bulkhead by initiating an explosion on the other side of the bulkhead, and transmitting the shock wave from the first explosion through the bulkhead. However, the use of an explosion and the resulting shockwave may be undesirable in many applications where the explosion and shockwave can damage equipment, or where an output of heat and/or flame is required.
Similarly, U.S. Pat. No. 4,503,773 discloses a through bulkhead initiator for use with a rocket motor. The initiator consists of a thin metal bulkhead with a small explosive charge on either side of the bulkhead. The first explosive charge is detonated by a confined detonating fuse, producing a shock wave that passed through the bulkhead without breaking the bulkhead. The shock wave then detonates the second explosive charge on the other side of the bulkhead, initiating combustion of a flame output charge.
Pyrotechnic, electronic, and mechanical initiators that control the time to function of propellant, pyrotechnic, and explosive devices are known as delays, and are frequently used to control functions of munitions, such as self-destruct and self-disable, and the propellant ignition time of a rocket or rocket assisted projectile, where the timing of the ignition of the propellant is critical in achieving maximum range. Pyrotechnic initiators that provide a delay time generally rely on the controlled burning of a pyrotechnic material, acting essentially as a fuse, such that the length of the column of pyrotechnic material and the burning rate of the material determine the time of the delay. That is, the delay time is the time between the ignition of the pyrotechnic column and the ignition of the propellant, pyrotechnic, and explosive device by the heat and/or flame output generated by the combustion of the pyrotechnic column. For example, in a projectile having a range extending propellant, the initial end of a pyrotechnic delay column/ignition train is ignited as the shell is fired. The range extending propellant grain is then ignited by the heat and/or flame output of the pyrotechnic delay column/ignition train when the burning portion of the delay column/ignition train reaches the propellant. The delay time is then the time between the ignition of the pyrotechnic delay column/ignition train and the ignition of the range extending propellant grain by the output of the pyrotechnic delay column/ignition train.
Such pyrotechnic initiators that provide delays typically require a rapid burn rate for reliability. Slower burning pyrotechnics are harder to ignite than fast burning pyrotechnics, and, typically, do not burn at a constant rate. Therefore, the delay time of slow burning pyrotechnics is less reliable than faster burning pyrotechnic delays, and reliable longer delay times are not easily obtained.
Control of the delay time of reliable, fast burning pyrotechnic delays is achieved by determining the burn rate of the pyrotechnic material and the length of pyrotechnic material that is needed to burn for the required time. As a result, the use of pyrotechnic delays in timing munition events is primarily limited by the space requirements of the munition, i.e., by the length of the column that will fit in the munition. Therefore, extended delay times are difficult to achieve because of the excessive length of pyrotechnic material required and/or the need for a slow burning pyrotechnic material. Typical size limitations for pyrotechnic delays using burn rate and column length to control the delay are driven by a nominal lower burn rate of about 0.1 inch (2.5 mm) per second for pyrotechnic columns having a cross section of about xe2x85x9 inch (3 mm) for columns up to about xe2x85x9c inch (9.1 mm), with cross-sections of about xc2xc inch (6.4 mm) for longer columns. The burn rate, heat loss, and column cross section are all closely interrelated, and, thus, the column must be carefully tailored to obtain reliable performance at or near the limits described above.
Electronic delays are typically used in situations where pyrotechnic delays are inadequate. The requirements for the self-destruction of munitions dictates long delay times, i.e., in excess of 30 seconds. For long delay times, electronic delay mechanisms are typically utilized because pyrotechnics cannot provide the delay time required within the packaging constraints. For time delays greater than 30 seconds, electronic delays offer greater packaging efficiency than pyrotechnic delays, but at a significant cost premium. In addition, electronic delays are much less durable than pyrotechnic delays, being comparatively fragile and, thus, susceptible to damage by the high acceleration or xe2x80x9cgxe2x80x9d loading experienced when the projectile is fired or the munition is launched or ejected.
Mechanical delays are less common for timing munitions because of their poor reliability. In particular, pre-wound spring mechanisms fatigue over time, and complex winding or other energizing mechanisms are inherently less reliable.
Although unique pyrotechnic initiators that utilize heat transfer through various media to provide a thermal output with a short delay time, i.e., less than about 0.5 seconds, are known in the art, there is no known disclosure of pyrotechnic initiators having a non-detonating thermal output that are capable of providing a reliable delay time of greater than 0.5 seconds. For example, U.S. Pat. No. 2,506,157 to Loret discloses a delay action blasting cap that allows a series of blasting caps to be produced having delays that differ one from another by small fractions of a second. The delay action blasting cap comprises an ignition charge in intimate contact with one end of a piece of heat conducting incombustible material, having an explosive fulminating element, i.e., a primary explosive that detonates upon ignition, at the end opposite the ignition charge. Upon combustion of the ignition charge, heat is transferred to, and travels through the piece of heat conducting material. The transfer of heat through the heat conducting material to the fulminating element causes the fulminating element to detonate, resulting in the detonation of the output charge. The amount of time required for the heat to travel from the ignition charge, through the heat conducting material to the fulminating element, causing the fulminating element to detonate, is the delay time of the blasting cap. However, the detonating output of such a delay is not as practical for initiating a propellant or pyrotechnic device that requires a heat and/or flame output.
U.S. Pat. No. 2,429,490 to Scherrer discloses detonators having delay times of from about 5 to about 30 milliseconds (ms). The delay is obtained by placing a thin metal disk, e.g., about 0.0015 inch thick, between a heating charge and a detonating charge. Heat generated by the combustion of the heating charge is rapidly transmitted through the disk to initiate the detonating charge after a short delay.
U.S. Pat. No. 3,727,552 to Zakheim discloses a bidirectional delay connector comprising a shell containing a separate detonating charge adjacent to each end of the connector, where the ends are adapted to receive a detonating fuse. Each detonating charge is also in close proximity to an exothermic charge at an end of a centrally located metal relay capsule, where a heat-conductive metallic delay element is positioned between each heat sensitive charge and the relay capsule containing the exothermic charges. Delay times on the order of 200 ms are produced.
U.S. Pat. No. 3,999,484 to Evans discloses a delay device having a dimpled transfer disc positioned between a delay charge and an output explosive charge. The delay time of 20 ms to 20 s is provided by the burning time of the delay charge. The disc, which is typically only about 0.01 inch thick, contributes little to the overall delay time.
U.S. Pat. Nos. 4,358,998 and 5,593,181 to Schneider et al. and Walker et al., respectively, disclose igniters for pyrotechnic gas bag inflators for vehicles, where short delays on the order of a few milliseconds are required.
A need exists for a small, reliable, low cost initiator or delay mechanism having a non-detonating thermal output. The present invention provides such a initiator.
The present invention is directed to a non-detonating heat transfer initiator and to a method of producing a non-detonating output with the initiator of the invention. The non-detonating heat transfer initiator of the invention comprises a heat transfer control medium, having a heat input portion and a heat output portion, and a non-detonating autoignition material, having an autoignition temperature, in thermal contact with the heat output portion, where the heat transfer control medium may be in the form of a housing or a thermal choke. Application of heat to the heat input portion causes a transfer of heat through the heat transfer control medium to the heat output portion, heating the heat output portion, such that, upon application of a sufficient amount of heat to the heat input portion, the heat output portion is heated to the autoignition temperature of the non-detonating autoignition material, igniting the non-detonating autoignition material ignites, thus producing a non-detonating thermal output. The non-detonating heat transfer initiator of the invention may further comprise a pyrotechnic heat source in thermal contact with the heat input portion as the source of heat applied to the heat input portion.
To at least partially reduce heat loss from the heat transfer control medium, the non-detonating heat transfer initiator may further comprise an insulating material at least partially surrounding the heat transfer control medium. Useful insulating materials include ceramics, filled epoxy resins, glasses, composites, paints, laminates, non-heat-conductive polymers, expanded polytetrafluoroethylene, natural and synthetic rubbers, urethanes, and heat resistant composites. Preferably, the insulating material is glass tape, polyethylene, an epoxy, or expanded polytetrafluoroethylene.
The non-detonating heat transfer initiator of the invention may take various forms, such as, e.g., a bulkhead, having first and second opposed side surfaces, where the first side surface serves as the heat input portion, and the second side surface serves as the heat output portion. The heat output portion may comprise a heat output source cavity defined in the second opposed side of the bulkhead. Optionally, the non-detonating heat transfer initiator may further comprise a pyrotechnic heat source in thermal contact with the heat input portion. In such a device, the heat input portion may further comprise an input heat source cavity defined in the first side surface of the bulkhead into which the optional pyrotechnic heat source is placed.
In an alternate embodiment, the non-detonating heat transfer initiator is in the form of a rod or disk, having first and second opposed surfaces, where the first surface serves as the heat input portion, and the second surface serves as the heat output portion. The heat output portion may comprise any of an output heat source cavity defined in the second opposed surface of the rod or disk, a pyrotechnic heat source in thermal contact with the heat input portion, and an input heat source cavity defined in the first surface of the rod or disk. The heat transfer control medium may serve as a thermal choke having a cross sectional area and a thermal conductivity that control the transfer of heat from the heat input portion to the heat output portion.
The non-detonating heat transfer initiator may be used as a through-bulkhead-initiator (xe2x80x9cTBIxe2x80x9d) by positioning the heat transfer control medium in an aperture defined by a bulkhead, having a first side and a second opposed side. Preferably, an insulating material partially surrounds the heat transfer control medium to at least partially reduce heat loss from the heat transfer control medium by forming at least a partial thermal barrier between the heat transfer control medium and the bulkhead. At least one of the heat input portion and the heat output portion may be substantially flush with the first side or the second opposed side of the bulkhead, or may extend outwardly from or may be depressed into the first side or the second opposed side of the bulkhead.
The invention also provides a method of producing a non-detonating thermal output. The method comprises applying heat to a heat transfer control medium in thermal contact with a non-detonating autoignition material, the non-detonating autoignition material having an autoignition temperature, conducting at least a portion of this heat through the heat transfer control medium to the non-detonating autoignition material, raising the temperature of the non-detonating autoignition material with the heat to at least the autoignition temperature, and, thus, igniting the non-detonating autoignition material, and producing a non-detonating thermal output due to the ignition. The method may further comprise insulating at least a portion of the heat transfer control medium to prevent heat loss. The method may further comprise placing a pyrotechnic heat source in thermal contact with the heat transfer control medium, igniting the pyrotechnic heat source, thereby producing heat from combustion or reaction of the pyrotechnic heat source, and transferring at least a portion of the heat from the combustion or reaction to the heat transfer control medium. Where a pyrotechnic heat source is not used, the source of heat may be the result of an increase in ambient temperature from, e.g., a fire or the like. As It will be recognized that
The autoignition material useful in the heat transfer initiator and the method of the invention is preferably non-detonating, and may be nitrocellulose, nitroglycerine based smokeless gun powders, safety and strike anywhere match compositions, smoke compositions, friction primer compositions, plastic bonded starter compositions, white smoke compositions, sugar based compositions, diazidodinitrophenol (DDNP) compositions, mixtures of an oxidizer composition and a powdered metal fuel, and mixtures thereof. Preferably, the non-detonating autoignition material comprises a mixture of an oxidizer composition and a powdered metal fuel, where the oxidizer composition is selected from the group consisting of alkali metal nitrates, alkaline earth metal nitrates, complex salt nitrates, dried, hydrated nitrates, silver nitrate, alkali metal chlorates, alkali metal perchlorates, alkaline earth metal chlorates, alkaline earth metal perchlorates, ammonium perchlorate, sodium nitrite, ammonium nitrate, potassium nitrite, silver nitrite, complex salt nitrites, solid organic nitrates, solid organic nitrites, solid organic amines, and mixtures and comelts thereof. Most preferably, the oxidizer composition is selected from the group consisting of silver nitrate, and mixtures and comelts of at least one of silver nitrate or ammonium nitrate and at least one of alkali metal nitrates, alkaline earth metal nitrates, ammonium nitrate, complex salt nitrates, dried, hydrated nitrates, alkali metal chlorates, alkali metal perchlorates, alkaline earth metal chlorates, alkaline earth metal perchlorates, ammonium perchlorate, nitrites of sodium, nitrites of potassium, nitrites of silver, solid organic nitrates, solid organic nitrites, and solid organic amines. The powdered metal fuel is preferably selected from the group consisting of molybdenum, magnesium, calcium, strontium, barium, titanium, zirconium, vanadium, niobium, tantalum, chromium, tungsten, manganese, iron, cobalt, nickel, copper, zinc, cadmium, tin, antimony, bismuth, aluminum, cerium, silicon, and mixtures thereof, and is most preferably molybdenum.
Representatives of the non-detonating autoignition material include mixtures of potassium nitrate, silver nitrate, and molybdenum; guanidine nitrate, silver nitrate, and molybdenum; silver nitrate, potassium nitrate, guanidine nitrate, fumed silica, and molybdenum; lithium nitrate, guanidine nitrate, ammonium perchlorate, fumed silica, and molybdenum; ammonium nitrate, guanidine nitrate, and molybdenum; mixtures of ammonium nitrate, guanidine nitrate, nitroguanidine, and molybdenum; mixtures of ammonium nitrate, tetramethylammonium nitrate, and molybdenum; mixtures of ammonium nitrate, guanidine nitrate, tetramethylammonium nitrate, and molybdenum; mixtures of ammonium nitrate, 5-aminotetrazole, potassium chlorate, and molybdenum; mixtures of ammonium nitrate, 5-aminotetrazole, potassium perchlorate, and molybdenum; mixtures of ammonium nitrate, barbituric acid, potassium chlorate, and molybdenum; and mixtures of ammonium nitrate, barbituric acid, potassium perchlorate, and molybdenum.
The pyrotechnic heat source may be a thermite, thermate, delay composition, halogenated composition, torch/flare composition, igniter composition, intermetallic composition, or mixtures thereof.
Useful materials for the heat transfer control medium include any material that will conduct heat from the pyrotechnic heat source to the autoignition material, including, but not limited to metals, alloys, ceramics, aluminas, silicas, alumina silicates, alumina borates, alumina silica borates, alumina nitrides, beryllias, carbides, composites, fiberglass, and graphite.
Preferably, the heat transfer control medium serves as a thermal choke having a cross sectional area and a thermal conductivity that control the transfer of heat from the heat input portion to the heat output portion of the heat transfer control medium. In addition, to reduce or eliminate a loss of heat from the heat transfer control medium, an insulating material at least partially surrounding the heat transfer control medium may be used. Useful insulating materials include, but are not limited to, ceramics, filled epoxies, glasses, composites, paints, laminates, non-heat-conductive polymers, expanded polytetrafluoroethylene (PTFE), such as GORE-TEX(copyright) and TEFLON(copyright), natural and synthetic rubbers, urethanes, and heat resistant composites, where glass tape, polyethylene, an epoxy resin, expanded TEFLON(copyright), or PTFE are preferred.
The present invention may be used to provide a delay time in propellant, pyrotechnic, and explosive devices. Upon the application of heat to the heat input portion of the heat transfer control medium, such as, e.g., from the ignition and combustion or reaction of a pyrotechnic heat source or an increase in ambient temperature, the heat is transferred through the heat transfer control medium to the heat output portion of the heat transfer control medium. When a sufficient amount of heat is applied to the heat input portion, the heat output portion is heated to a temperature sufficiently high to ignite the non-detonating autoignition material, and produce a non-detonating thermal output therefrom, where the heat transfer control medium conducts heat at a rate. In this manner, a delay time of at least about 0.5 second can be obtained between the application of heat and the ignition of the non-detonating autoignition material. By varying at least one parameter, such as, e.g., the cross sectional area, length, and thermal conductivity of the heat transfer control medium, the amount of heat applied, which may be determined from the ambient temperature or the amount and heat of reaction of the pyrotechnic heat source, and/or the autoignition temperature of the non-detonating autoignition material, the delay time can be adjusted to a desired duration, such as, e.g., at least about 0.5, 1, 2, 5, 10, 15, 20, 30, 60, 90 seconds, or longer.
The invention also provides a method of delaying production of a non-detonating thermal output using the heat transfer initiator of the invention. The method comprises placing a heat transfer control medium, which may be insulated, in thermal contact with a heat source and a non-detonating autoignition material. The heat provided by the heat source is conducted through the heat transfer control medium to the non-detonating autoignition material, raising the temperature of the non-detonating autoignition material to at least the autoignition temperature of the material, and, thus, igniting the non-detonating autoignition material, and producing a non-detonating thermal output due to the ignition. Preferably, the heat transfer control medium conducts heat at a rate such that a delay time of at least about 0.5 seconds elapses between ignition of the heat source and ignition of the non-detonating autoignition material. At least one of the cross sectional area, length, and thermal conductivity of the heat transfer portion, the amount and heat of reaction of the pyrotechnic heat source, the autoignition temperature of the non-detonating autoignition material may be varied to adjust the delay time to a desired duration.